Process for preparing 1,1,1,2-tetrafluoroethane

ABSTRACT

1,1,1,2-tetrafluoroethane (134a) is prepared by reacting, in the gas phase, trichloroethylene with 1,1,1-trifluorochloroethane (133a) and hydrofluoric acid with trichloroethylene/133a molar ratios ranging from 5/95 to 50/50, in the presence of a catalyst consisting of Cr 2  O 3  carried on AlF 3 . 
     The process provides 134a yields higher than 90% and permits an exceptionally long life of the catalyst. In this way it is possible to realize a continuous process by recycling the unreacted trichloroethylene and 133a, thereby making up for the relatively low global conversion of the reagents.

This is a continuation, of U.S. application Ser. No. 08/752,685, filedNov. 19, 1996, now U.S. Pat. No. 5,880,316, which is a continuation ofU.S. application Ser. No. 08/507,644, filed Jul. 25, 1995, now U.S. Pat.No. 5,608,125, which is a continuation of U.S. application Ser. No.08/337,128, filed Nov. 10, 1994, now U.S. Pat. No. 5,463,151, which is acontinuation of U.S. application Ser. No. 08/191,765, filed Feb. 3, 1994(now abandoned), which is a continuation of U.S. application Ser. No.07/928,188, filed Aug. 14, 1992 (now abandoned), which is a continuationof U.S. application Ser. No. 07/811,920 filed Dec. 23, 1991 (nowabandoned), which is a continuation of Ser. No. 07/550,559, filed onJul. 10, 1990 (now abandoned).

DESCRIPTION OF THE INVENTION

The present invention relates to a process for preparing, in the gasphase, 1,1,1,2-tetrafluoroethane by reaction of trichloroethylene(CHCl=CCl₂) with hydrofluoric acid in the presence of catalysts.

It is known that it is possible to obtain 1,1,1,2-tetrafluoroethane(hereinafter referred to as 134a) by catalytic reaction in the gas phasebetween 1,1,1-trifluorochloroethane (hereinafter referred to as 133a)and hydrofluoric acid according to the reaction: ##STR1##

According to U.S. Pat. No. 4,129,603, said reaction is made to occur byusing a catalyst consisting of chrome oxide or, at least partially, ofbasic chrome fluoride, at temperatures ranging from 300° to 400° C.,thereby obtaining 133a conversions of 20%, with yields of 134a equal to91%.

According to said process,also CF₂ =CHCl forms (in consequence of 133adehydrofluorination), the boiling point of which, which is higher byonly 9° C. than the boiling point of 134a, makes the separation thereofdifficult and anti-economic.

Thus, Belgian patent No. 863,913 describes a method for reducing oreliminating the abovesaid impurity from the reaction products, whichconsists in the olefin post-fluorination carried out at low temperaturewith the same type of catalyst.

According to another method, described in Canadian patent No. 1,124,265,such olefin content is reduced to 5-10 ppm by contacting the 133afluorination products with an aqueous solution of KMnO₄.

Other processes for preparing 134a by means of fluorination of 133a withcatalysts based on chrome oxide are described in Japan patentapplication No. 80-27138 and in German patent application No. 2,932,934,according to which applications conversions of 21% and 31%, with 134ayields of 91% and 98%, are respectively obtained.

European patent No. 300,724 described a process for preparing in theliquid phase, 134a by fluorination of 133a in the presence of catalystsbased on antimonium halides. Said process is affected by the drawback ofnot being highly selective, in particular owing to the considerablepentafluoroethane amount produced. In said process, mention is made ofthe difficulty to produce 134a through fluorination, in the gas phase,of trichloroethylene.

In fact, the Applicant has ascertained by means of tests, that thedirect fluorination of trichloroethylene in the gas phase using, forexample, a chrome oxyfluoride catalyst, produces 134a with yields ofonly 3%, although it provides high yields of 133a and trichloroethyleneconversions of 92%.

Furthermore, the catalyst activity rapidly decays, wherefore such aprocess, if it were utilized to produce 134a with acceptable yields,would require frequent reactivations of the catalyst, to the prejudiceof its applicability on an industrial scale.

An alternative would be that of preparing separately 133a byfluorination of trichloroethylene in the liquid phase and of using itfor preparing 134a through fluorination in the gas phase.

This solution, however, would result in a considerable industrialburden, as two distinct plants with two different technologies would berequired.

The Applicant has now found that it is possible to realize an industrialprocess in the gas phase for preparing 134a with industrially acceptableconversions, the process being furthermore highly selective, by reactinga mixture of trichloroethylene and 133a in trichloroethylene/133a molarratios ranging from 5/95 to 50/50 with hydrofluoric acid, in thepresence of a catalyst consisting of Cr₂ O₃ carried on aluminiumtrifluoride.

In such process, which is the object of the present invention, thecatalyst surprisingly retains its full activity for very long stretchesof time, of the order of hundreds of hours, thereby permitting torealize the process on an industrial scale, continuously.

A preferred mode of carrying out the abovesaid process consists infeeding, at the beginning, the reactor containing the catalyst with thetrichloroethylene and 133a mixture along with hydrofluoric acid in theabove-mentioned ratios, in separating, at the reactor outlet, the 134a,which has formed, from the other reaction products, prevailinglyconsisting of 133a, and in recycling said products to the reactor afteraddition of trichloroethylene and hydrofluoric acid in order to restorethe reagents in the above-indicated ratios.

The reaction among trichloroethylene, 133a and hydrofluoric acid ispreferably conducted at temperatures from 300° to 400° C., and even morepreferably at temperatures from 330° to 380° C., at atmospheric pressureor also at higher pressures, up to 15 atmospheres.

Preferred feeding conditions utilize trichloroethylene/133a molar ratiosof about 15/85. Generally it is operated with HF/trichloroethylene+133amolar ratios not lower than 3, while particularly critical upper valuesof such ratio do not exist. However, it is preferable to operate withHF/trichloroethylene+133a molar ratios ranging from 3/1 to 10/1 and,even more preferably, from 4/1 to 6/1.

The contact time between reagents and catalyst is not critical above aminimum threshold of one second. Usually it is operated with contacttimes ranging from 1 second to 50 seconds,and preferably from 5 to 20seconds.

The process can be conducted both discontinuously, collecting thereaction products after only one run on the catalyst, and continuously,recycling the unreacted trichloroethylene and the 133a to the reactor,after having restored the reagents amount in the above-defined ratios,as mentioned hereinbefore.

The catalyst to be used in the process of the invention is composed, asmentioned above, of chrome trioxide carried on AlF₃ in the gamma and/orbeta form.

The Cr₂ O₃ amount generally ranges from 1 to 15% by weight, calculatedas Cr on the carried catalyst. The per cent of Cr₂ O₃ is a function ofthe surface area of AlF₃ in the gamma form.

Carriers having a great surface area, of the order of 25-30 m² /g, aregenerally preferred.

The carrier can be in the form of powders having particle sizesgenerally in the range of from 20 to 200 microns; if necessary, however,it can be also in the form of pellets.

AlF₃ may comprise, besides the gamma and/or beta form, also the deltaform, generally also up to amounts of 30% by weight.

Also AlF₃ in the alpha form can be present, although it is preferablethat the amount thereof should be limited, as this form has proved to belittle active.

The catalyst of the invention can be prepared in various manners, one ofthe preferred methods being the following: the AlF₃ carrier in theabove-cited crystallographic forms is impregnated, according to one ofthe conventional techniques of the art, in wet conditions or in dryconditions, with a solution of a soluble salt of trivalent chrome, forexample CrCl₃.6H₂ O.

The catalyst is then dried in order to remove the water present therein,then it is charged into a reactor and is subjected to an activationtreatment with air or nitrogen either or not in the presence of steamand/or of crystallization water, which can act as an oxidant.

The activation treatment is generally carried out at temperaturesranging from 200 to 600° C., preferably from 350° to 500° C., in orderto convert chrome into the oxide form.

The above-cited allotropic structures of AlF₃ are known and arecharacterized by means of the X-ray diffraction spectrum, as isindicated, for example, in J.C.P.D.S. 1981 and in French patent No.1,383,927 to Du Pont.

The above-considered gamma_(C), delta_(C) and beta_(C) phases are theones described in French patent No. 1,383,927 by J. Cristoph and J.Teufer. The alpha phase is described in Anal. Chem. 29, 984 (1957).

After a long-lasting use, the catalytic activity can be reactivated bymeans of an air treatment at high temperatures (from 350° to 500° C.).

EXAMPLES

The following examples are given to illustrate the object of the presentinvention, without being, however, a limitation thereof.

Example 1 Catalyst Preparation

Into an Inconel tubular reactor having a diameter of 8 cm and a lengthof 100 cm, electrically heated and equipped with a sintered Inconelporous baffle, 1680 g of a catalyst prepared as described hereinafterwere charged.

A carrier consisting of AlF₃, prevailingly in the gamma form and havinga specific surface of 26 m² /g, was impregnated with an aqueous solutionof CrCl₃.6H₂ O in the rate of 492 g of CrCl₃.6H₂ O per kg of AlF₃.

The solution, consisting of 492 g of CrCl₃.6H₂ O+152 ml of H₂ O, had avolume of 450 ml and was added to AlF₃ in three almost equal portions.After each addition, the catalyst was dried for 4 hours at 120° C. atatmospheric pressure. After the third drying, the catalyst was alsosieved and charged into the reactor.

The catalyst was fluidized with a nitrogen stream (about 100 l/h) for 10hours in the reactor heated to 400° C., then the reactor was brought tothe operating temperature.

Example 2 Comparative Test

The above-described reactor was fed, at 380° C., with 1.536 moles/h oftrichloroethylene and 9.137 moles/h of anhydrous HF, obtaining a HF/C₂HCl₃ molar ratio equal to 6 and a contact time of 9.6 seconds,calculated as a ratio between non-fluidized catalyst ratio andvolumetric flowrate of the reagents at the reaction temperature andpressure (the pressure was slightly higher than the atmosphericpressure). The gases leaving the reactor were drawn for 1 hour; afterabsorption of HCl and HF in water and washing of the reaction productwith a NaOH aqueous solution, 182 g of a product were recovered, themolar composition of which was as follows:

    ______________________________________                                               CF.sub.3 CH.sub.2 Cl                                                                 87.9%                                                             CF.sub.3 CH.sub.2 F 2.0%                                                      C.sub.2 HCl.sub.3 4.9%                                                      ______________________________________                                    

The balance was prevailingly composed of CF₃ CHF₂ and CF₃ CH₃. Theconversion of C₂ HCl₃ was of 95.1% and the selectivity in CF₃ CH₂ Cl wasof 92.4%, while the selectivity in CF₃ CH₂ F was of 2.1%. These resultswere obtained, without any variations, for about 50 hours of run,whereafter the catalyst activity began to decay.

Example 3

The above-described reactor was fed, at 350° C. and at a slightly higherpressure than the atmospheric pressure, with 0.072 moles/h of C₂ HCl₃,0.62 moles/h of CF₃ CH₂ Cl and 3.814 moles/h of anhydrous HF, soobtaining a contact time of 19.5 seconds, a HF/organic product molarratio equal to 5.5 and an amount of C₂ HCl₃ on the total organic productequal to 10.4%.

Operating as in example 2, 80 g of a product were recovered, the molarcomposition of which was as follows:

    ______________________________________                                               CF.sub.3 CH.sub.2 Cl                                                                 82.1%                                                             CF.sub.3 CH.sub.2 F 16.5%                                                   ______________________________________                                    

The balance consisted of little amounts of CF₂ ═CHCl, CHCl═CCl₂, CF₃CHF₂ and CF₃ CH₃. The trichloroethylene conversion was almostquantitative, the total conversion was of 17.7% and the selectivity inCF₃ CH₂ F was equal to 93.2%.

Example 4

Into the above-described reactor and under the conditions of example 3,0.194 moles/h of C₂ HCl₃, 1.010 moles/h of CF₃ CH₂ Cl and 7.403 moles/hof anhydrous HF were charged, thereby obtaining a contact time of 10.2seconds, a HF/organic product ratio of 6.2 and a C₂ HCl₃ amount on thetotal organic product equal to 16.1%.

Operating in like manner as the preceding examples, 140 g of a productwere obtained, the molar composition of which was as follows:

    ______________________________________                                               CF.sub.3 CH.sub.2 Cl                                                                 85.0%                                                             CF.sub.3 CH.sub.2 F 13.6%.                                                  ______________________________________                                    

The by-products were the same as in example 3. The trichloroethyleneconversion was almost quantitative, the total conversion was equal to14.5% and the selectivity in CF₃ CH₂ F was of 93.8%.

Example 5

Into the above-described reactor and under the conditions of example 3there were charged 0.250 moles/h of C₂ HCl₃, 0.714 moles/h of CF₃ CH₂ Cland 7.468 moles/h of anhydrous HF, thereby obtaining a contact time of10.4 seconds, a HF/organic product ratio equal to 7.8 and a C₂ HCl₃amount on the total organic product equal to 25.9%.

Operating in like manner as in the preceding examples it was possible toobtain 112 g of a product, the molar composition of which was asfollows:

    ______________________________________                                               CF.sub.3 CH.sub.2 Cl                                                                 86.9%                                                             CF.sub.3 CH.sub.2 F 11.8%                                                   ______________________________________                                    

The by-products were the same as in example 3. The trichloroethyleneconversion was almost quantitative, the total conversion being equal to12.9% and the selectivity in CF₃ CH₂ F being equal to 91.5%. Under theseconditions as well as under the ones of the preceding examples,thecatalyst activity did not exhibit an appreciable decay after about a200-hour run.

Example 6

The reactor of the preceding examples was converted into a plant capableof operating continuously, by addition of a separation column where thelight products, including 134a (CF₃ CFH₂), were drawn, while CF₃ CH₂ Cland the higher-boiling products were conveyed to a pump and fed again tothe reactor, along with fresh trichloroethylene and HF, in order to makeup for the consumptions.

The amount of fresh make-up products underwent slight variations in thetime in order to maintain constant the trichloroethylene/CF₃ CH₂ Cl andHF/organic product ratios. The reference conditions were:

reaction temperature=350° C.; contact time=10 seconds; HF/organicproduct ratio=6; trichloroethylene/CF₃ CH₃ Cl ratio=15/85 at the reactorinlet.

After the plant had been adjusted to the operating conditions with atrichloroethylene/CF₃ CH₂ Cl mixture prepared in advance, there werefed, in 6 hours, 1.08 moles of fresh trichloroethylene and 4.50 moles offresh anhydrous HF, while about 1 mole of CF₃ CH₂ F and little amountsof by-products of the same nature as the ones cited in the precedingexamples were drawn from the top of the distillation column.

What is claimed is:
 1. A process for the preparation of1,1,1,2-tetrafluoroethane comprising the steps:(a) reacting, in the gasphase in a reaction zone, trichloroethylene with1,1,1-trifluorochloroethane and HF, operating withtrichloroethylene/1,1,1-trifluorochloroethane molar ratios ranging from5/95 to 50/50, with HF/(trichloroethylene+1,1,1-trifluorochloroethane)molar ratios of at least 3/1, at a temperature of from 300° C. to 400°C., at a pressure of up to 15 atmospheres, and in the presence of acatalyst comprising Cr₂ O₃ carried on AlF₃ in the gamma and/or betaform, to produce reaction products including 1,1,1,2-tetrafluoroethane;and (b) continuously recycling to the reaction zone the unreactedtrichloroethylene and the 1,1,1-trifluorochloroethane from the reactionproducts leaving the reaction zone and continuously restoring thereagent molar ratios defined in step (a) by the addition to the reactionzone of fresh trichloroethylene and HF.
 2. The process of claim 1wherein the reaction is carried out at a temperature of from 330° C. to380° C.
 3. The process of claim 1 wherein the contact time of thereagents ranges from 1 to 50 seconds.
 4. The process of claim 1 whereinthe contact time of the reagents ranges from 5 to 20 seconds.
 5. Theprocess of claim 1 wherein the AlF₃ exhibits a specific surface area offrom 25 to 30 m² /g.
 6. The process of claim 1 wherein the AlF₃comprises principally the gamma form.
 7. The process of claim 1 whereinthe AlF₃ further comprises the delta form.
 8. The process of claim 1wherein the catalyst is prepared by impregnating AlF₃ comprising AlF₃ inthe gamma and/or beta form with a solution of a soluble salt oftrivalent chromium, drying, and activating the catalyst with air ornitrogen at 200° to 600° C.
 9. The process of claim 1 wherein theactivity of the catalyst does not appreciably decay during at leastabout 200 hours of use in the process.